A big event is looming over the ancient riverside town of Henley. But it is not Thursday's byelection to succeed Boris Johnson as MP for the town and its gorgeous, well-heeled rural hinterland. The big one is the annual regatta - and local shops are ready for the starter's pistol on July 2. There is a solitary Tory poster on Henley bridge.

But the byelection is trundling along towards its predictable conclusion. In choosing to stage it quickly after mayor Boris stepped down, David Cameron has closed off the only potential upset scenario, a Liberal Democrat insurgency.

Things might have been different if Johnson's colourful tenure as MP since 2001, or the manner of his departure for bigger things, had offended Henley. On the contrary, he seems to be remembered with affection.

So much so that the Lib Dems last week published an election flyer in which Boris's photo is artfully included twice in such a way that innocent voters might think he is endorsing Nick Clegg's candidate, Stephen Kearney.

Demanding its withdrawal, London's mayor denounced another "shameful exercise in misrepresentation" by his second-placed local rivals, who claim that - at one time or another - most council wards in the constituency have fallen their way. However, Johnson's majority in 2005 was a hefty 12,793.

Both main contenders have been accusing each other of dirty tricks, for instance printing leaflets with no clear party label (both are guilty), or unfairly claiming credit for helping save the 120-bed Townland hospital from closure. Tory candidate and local county councillor John Howell is said to have done that.

According to the Lib Dems, Howell gave advice to developers who want to build too much in South Oxfordshire's green and pleasant villages. According to the Tories, Kearney's campaign has been making "scurrilous" and inaccurate charges about budget cuts and poor results in local schools.

Before making his fourth visit so far in the campaign on Friday morning (it is ghandily en route to his own adjacent patch at Witney), Cameron did an early turn for BBC Radio Berkshire to denounce the Lib Dems' "deeply negative and disgraceful campaign".

He even threatened legal action. "The Tories should be playing down by-election stories, not building them up. It shows they're running scared. People are saying, 'We used to vote for Boris, he was a good MP. We're not sure about the new chap'," counters Chris Leaman, the Lib Dems' campaign organiser.

But it will take more than routine political knockabout to disturb the prosperous calm of Henley, where the average price of a house is £820,000 and "top-hatted toffs" of the kind that disfigured Labour's campaign in Crewe are a normal part of the scenery.

The Henley Standard ("Big Guns in Town") is suitably sceptical about such claims and some voters passing the handsome Queen Anne town hall seem stuck for something to complain about. "Rowdy teenagers look threatening late at night," admits one well-dressed pensioner, before spoiling it by adding: "I'm sure they're not really."

Dig a little harder and the visitor finds worries about unsuitable building developments, traffic jams in narrow streets, the risk of pub, shop and GP surgery closures in villages and the rising cost of fuel in rural areas where cars are a necessity. Cameron plays on them skilfully.

As for First Great Western's branch line rail service taking commuters to London, it is sometimes "so bad that the passengers went on strike recently," he told students at the 880-strong Gillotts comprehensive school on the outskirts of town.

In truth there are pockets of poverty in places like Berinsford, but Henley has few grounds for complaint. So Labour's Richard McKenzie, a marketing man from Reading, is going through the motions. So are the nine minor candidates, who range from Ukip and the BNP to Harry the Bear and two Miss Great Britains - blonde and brunette.

Running much harder is Kearney. Before his selection in May he lived 200 miles away in Plymouth. But local reporters rate him for his energy and accessibility. Last week he drew an audience of 200 to a meeting in Dorchester. Clegg, whose parents live nearby, has visited six times.

Ministers and shadow ministers have poured in too. "Henley has prospered under the Labour government," Harriet Harman told the Henley Standard. At Gillotts school Cameron stresses local decision-making ("that's what politics are all about") along with his reen credentials. They include decent feed-in-tariffs for householders who generate their own energy. Is it happening here? asks a student. "No, because we have a useless government. We need a new one, LED BY ME," he replies, slapping the desk in mock solemnity.

At his leader's side, Howell - uncharismatic but self-confident and articulate - does not attempt to compete. But he looks a safe enough pair of hands to join the backbench Tory ranks on Thursday if Cameron's run of luck holds.